allow to avoid presence notifications, but keep buddies in buddy list

[hannesm]

related to #22

the scenario as it unfolds: avoid presence notifications. this implies that there are no subscriptions (from/both) in the roster.

but nevertheless, these buddies should be shown (as ?_?) in the buddy list -- at the moment this is not always the case (todo: also check exactly when this happens and fix it).

the tradeoff / issue is that this allows DoS of your buddy list... c'est la vie -- maybe a configuration option to disable showing non-subscribed users (or the F5/offline stuff is sufficient).

[hannesm]

the implication is: add a new buddy each time a message from that buddy is received (and don't report the "presence error" / ... in the log)

[hannesm]

another feature request is "/msg message" to contact without any subscription

[hannesm]

ok, turns out: there are people not in the roster, but <presence> messages are received (with "subscription request")...